I'd say that the solution to your problem would be solved by bringing the new fish back. Reason the new fish being ... is that your original fish has been with you longer and is adjusted and happy in his tank. It seems to be unfair to bring him back to the LFS just because he is doing something that is only natural. He is defending his tank against another male.
By getting another gourami ( who more than likely will be another male) after returning your gourami is only prolonging the problem, and just replacing another gourmai.. with another gourami. IMO that just doesnt solve the problem... I find that the gouramis.. like their cousins the Bettas are happier by themselves if you have males.

~ kate